Segment Information - Reportable Segment Information (Reconciliation of Total Segment Operating Income to Consolidated Operating Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues	$ 1,117,838	$ 874,255	$ 720,815
Direct costs by segment	843,783	612,822	480,373
Income from operations	245,708	242,566	199,382
Global operating costs	28,404	19,013	41,257
Cloud Services
Segment Reporting Information [Line Items]
Total revenues	578,956	566,938	504,638
Direct costs by segment	352,912	356,059	294,436
Income from operations	226,044	210,879	210,202
Digital Media
Segment Reporting Information [Line Items]
Total revenues	538,939	307,463	216,374
Direct costs by segment	490,871	256,763	185,937
Income from operations	48,068	50,700	30,437
Elimination of inter-segment revenues
Segment Reporting Information [Line Items]
Total revenues	(57)	(146)	(197)
Reportable segments
Segment Reporting Information [Line Items]
Income from operations	$ 274,112	$ 261,579	$ 240,639